1. Field of the Invention
The invention relates in general to protective relay apparatus for electrical power systems, and more specifically to protective relay apparatus for detecting and clearing single-phase-to-ground faults regardless of fault impedance.
2. Description of the Prior Art
A conventional distribution substation for supplying three-phase and single-phase loads includes a three-phase circuit breaker equipped with phase overcurrent relays, and sometimes a ground-current relay. Reclosers and fuses complete the protection of the distribution circuit. Coordination is necessary between these protective devices in order to isolate only the required portion of the distribution circuit when a fault occurs. Because of normal circuit load unbalances, as well as normal cold-load pickup and switching surges, a ground relay, if used, must be set quite high in order to prevent nuisance tripping of the circuit breaker. Broken conductors close to or touching the ground through high impedance paths may not produce enough current or circuit unbalance to operate the phase overcurrent protective devices, or the ground relay.
Thus, it would be desirable to provide a new and improved protective relay which will detect such high impedance single-phase-to-ground faults. Such a relay, however, must meet the stringent requirements of the electric utility industry, including sensitivity, discrimination, reliability, security, low cost, ruggedness, relative simplicity, easy to apply and install, low maintenance, and it must be capable of properly coordinating with other protective schemes.